Looking at the TCP/IP stack, it seems that you are not allow to bypass any of the layers. For instance, you can't go from application layer straight to layer 3 (network).
So, let's say we have two routers connected to each other. Would you describe it as a Layer 3 infrastracture? Or would you also call it a Layer 2?
I'm saying Layer 2 also since the IP packets have to be encapsulated to a Layer 2 frame before it gets sent out to the other router via a, for example, Ethernet interface.